1. Field of the Invention
This invention relates to a novel amorphous silicon semiconductor film. More particularly, the invention relates to an amorphous silicon semiconductor film which has a low light absorptivity and is suitable for use as a window material, and a process of producing such a film.
2. Description of the Prior Art
Amorphous solar cells of a p-i-n structure have drawn the attention of researchers recently because of their extremely low production cost, compared with monocrystalline solar cells, and the development of such amorphous solar cells is being promoted eagerly, because they are now considered to be the most promising type of solar cells.
It is generally known that when an amorphous silicon semiconductor of a p-i-n structure is adapted to function as a solar cell, the carriers generated by the absorption of light by amorphous silicon (these carriers are hereinafter called light-generated carriers) in the p-layer or the n-layer have a short life, and therefore light-generated carriers which can provide a photocurrent are generated essentially in the i-layer. However, in a p-i-n type of solar cell, since the i-layer is sandwiched between the p-layer and the n-layer, light enters the i-layer after passing through the p-layer or n-layer, so that it is desirable that the p-layer or the n-layer be made of a material with a low light absorptivity. The light absorption coefficient of conventional n-type or p-type amorphous silicon is substantially equal to or greater than that of the amorphous silicon of the i-layer, so that light absorption losses in the n layer or the p-layer are one of the causes of lowering of the photoelectric conversion efficiency.
As a p-i-n type amorphous solar cell has a structure consisting of either (glass/transparent electrode/p-i-n or n-i-p/metal), or (transparent electrode/p-i-n or n-i-p/metal), light enters through the transparent electrode. In this structure, light losses due to the reflection of light by the solar cell are also a cause of reduced photoelectric conversion efficiency. In this case, in order to minimize the reflection of light at the interface between the transparent electrode and the p-layer or the n-layer, it is desirable to control the refractive index of the p-layer or the n-layer. In practice, however, the refractive index of the conventional p-layer or n-layer, for instance, is in the range of 3.4 to 3.6 for light of 1500 nm, the same as that of the amorphous silicon of the i-layer, which is substantially different from the refractive index of the transparent electrode, approximately 2.0, and because of this difference in refractive indices in the cell structure, reflection losses have been considered inevitable.
Another disadvantage of conventional p-i-n type amorphous silicon semiconductors is that their production yield is low because of the unsatisfactory adhesion of a p-i-n or n-i-p assembly to a stainless steel substrate and transparent electrode.
Amorphous silicon containing carbon and hydrogen as impurities (a-Si:C:H) has been proposed as a material which is free of these drawbacks, (Japan Patent Publication OPI-No. 126,175/82, etc.). This amorphous silicon (a-Si:C:H) can control light absorption and, in this respect, can help to improve the photoelectric conversion efficiency of a cell, but it is still unsatisfactory from the point of view of improvements in light reflection and adhesion to a stainless steel substrate and transparent electrode.
As a result of extensive studies into overcoming these drawbacks of the prior art, the present inventors have found that a thin film of a p-type or n-type amorphous silicon semiconductor obtained from the plasma decomposion of a mixture consisting of at least one gas selected from silane or its derivatives and fluoro silane or its derivatives, and at least one one gas selected from CO, CO.sub.2, oxygen-containing organic compounds, and mixtures of hydrocarbons and oxygen, contains at least hydrogen, carbon, and oxygen as impurities (this amorphous silicon semiconductor is hereinafter referred to as a-Si:H:C:O), and that it has a small light absorption coeficient. Moreover the refractive index of thus-obtained a-Si:H:C:O can be controlled, so that it is ideal for use as a window material for photoelectric converter elements with p-i-n junctions. The present invention was achieved on the basis of this novel finding.